Epigenetic silencing of microRNA-137 enhances ASCT2 expression and tumor glutamine metabolism

J Dong,B Li,H Liu,Y Hu,C Li,J Shi,P Ren,Z Zhao,L Wang,P Gao,H Su,D Xiao,G Qing

doi:10.1038/oncsis.2017.59

Abstract

Tumor cells must activate specific transporters to meet their increased glutamine metabolic demands. Relative to other glutamine transporters, the ASC family transporter 2 (ASCT2, also called SLC1A5) is profoundly elevated in a wide spectrum of human cancers to coordinate metabolic reprogramming and malignant transformation. Understanding the molecular mechanisms whereby tumor cells frequently upregulate this transporter is therefore vital to develop potential strategies for transporter-targeted therapies. Combining in-silico algorithms with systemic experimental screening, we herein identify the tumor suppressor microRNA, miR-137, as an essential regulator that targets ASCT2 and cancer cell glutamine metabolism. Metabolic analysis shows that miR-137 derepression, similar to ASCT2 inactivation, significantly inhibits glutamine consumption and TCA cycle anaplerosis. Mechanistically, methyl-CpG-binding protein 2 (MeCP2) and DNA methyltransferases (DNMTs) cooperate to promote active methylation of the miR-137 promoter and inhibit its transcription, conversely reactivating ASCT2 expression and glutamine metabolism. Moreover, expression between miR-137 and ASCT2 is inversely correlated in tumor specimens from multiple cancer types, and ectopic ASCT2 expression markedly rescued miR-137 suppression of tumorigenesis. These findings thus elucidate a previously unreported mechanism responsible for ASCT2 deregulation in human cancers and identify ASCT2 as a critical downstream effector of miR-137, revealing a molecular link between DNA methylation, microRNA and tumor metabolism.

Highlights

Elevated glutamine metabolism is an essential feature of malignant transformation
To investigate miRNAs potentially implicated in ASCT2 regulation, we performed in-silico analysis using the TargetScan, miRanada and DIANA algorithms, and identified a total of 18 miRNAs as predicted to target the 3′untranslated region (UTR) of ASCT2 mRNA (Figure 1a)
Since miR-137 functions as a global tumor suppressor miRNA in a wide spectrum of human cancers whereas miR-122 is a liverspecific miRNA downregulated in hepatocellular carcinoma,[28,29,30,31,32,33,34] we focused on miR-137 regulation of ASCT2 in the current study

Summary

Introduction

Elevated glutamine metabolism is an essential feature of malignant transformation. The importance of glutamine as a global, critical nutrient in fueling proliferation and survival has become better understood and appreciated in recent years.[1,2,3,4]Glutamine provides metabolic intermediates to replenish the tricarboxylic acid (TCA) cycle and maintain the mitochondrial integrity and nicotinamide adenine dinucleotide phosphate (NADPH) levels.[5,6,7,8,9,10,11,12,13,14,15,16] In addition, its carbon skeleton can be incorporated into glucose and fatty acids while the nitrogen part is used in the biosynthesis of purines and pyrimidines.[5,8,9,10,16,17] As such, glutamine metabolism provides cancer cells building blocks into an array of growth-promoting pathways, and alleviates them from wicked cellular microenvironment by maintaining a proper redox homeostasis. The ASC family member ASCT2, a high-affinity glutamine transporter, is receiving increasing attention for its critical roles in mediation of glutamine-dependent tumor cell growth, mammalian target of rapamycin (mTOR) activation and drug resistance.[21,22,23,24] Genetic depletion or pharmacological inhibition of ASCT2 results in growth repression and apoptosis in multiple human cancer types.[18,23,24] ASCT2 functions as a Na+-dependent transporter for glutamine as well as alanine, serine and cysteine.[23,24] The transport process is electroneutral, involving the Na+-coupled influx of glutamine to the Na+-coupled efflux of another substrate such as alanine, serine or cysteine, contributing to the homeostasis of amino acid metabolism within tumor cells Glutamine influx is mediated by three major families of transporter systems, system A, ASC and N.18,19 Among which, the ASC family member ASCT2, a high-affinity glutamine transporter, is receiving increasing attention for its critical roles in mediation of glutamine-dependent tumor cell growth, mammalian target of rapamycin (mTOR) activation and drug resistance.[21,22,23,24] Genetic depletion or pharmacological inhibition of ASCT2 results in growth repression and apoptosis in multiple human cancer types.[18,23,24] ASCT2 functions as a Na+-dependent transporter for glutamine as well as alanine, serine and cysteine.[23,24] The transport process is electroneutral, involving the Na+-coupled influx of glutamine to the Na+-coupled efflux of another substrate such as alanine, serine or cysteine, contributing to the homeostasis of amino acid metabolism within tumor cells

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Conclusion